reversible pump or fill and drain? you could split the lines 3 ways and get it done, would take forever though. if you have fill and drain pumps, you need to repeat the process to drain unless you want to drain to bilge.
you've basically tripled your fill time. you need to add a fill pump and thru-hull at a minimum. I'd also add a drain pump and avoid using the bilge pump so heavily. to do it right, you need 2 fills and 2 drains, 1 of each for each tank. will give you better fill time and much more control.
Very easy to do with a little homework.
get brass thru hull like this:
https://jet.com/product/detail/b2849...B&gclsrc=aw.ds
reversible pumps make install a tad easier, flow rate goes down and cost goes up with reversibles.